The present invention relates to a wireless handset using a slot antenna. More specifically, it relates to a wireless handset for use in a communication system switching a plurality of call frequencies and employing different transmit/receive frequencies, in particular, a wireless handset mounting a slot antenna.
Wireless handsets have been made smaller and thinner in order to improve portability, and at the same time, various small antennas mounted on such wireless handsets have been developed. Among them, a slot antenna utilizing a coaxial resonator can be incorporated without any protrusion, and in particular, Japanese Non-examined Patent Publication No. 9-74312 proposes a coaxial resonant slot antenna which is made smaller in which its central conductor (power supply conductor) is not in contact with a resonator case (conductive cubic). This coaxial resonant slot antenna is a magnetic current type antenna, and there is an influence between the magnetic current generated in the slot and a magnetic current in the same phase generated on a surface opposite a main surface provided with the slot in the conductive cubic, so as to realize single-side directional. Since an electromagnetic wave having frequencies above about 2 GHz can easily be absorbed into the head of the user in a call state of the handset, a single-side directional antenna which allows no electric power to be radiated to the user side is used to reduce power consumption. Similarly, in respect of received power, the single-side directional also increases antenna gain on the opposite side of the head to enhance handset sensitivity.
Since the antenna includes a resonator construction, the volume is directly proportional to the bandwidth. Thus, when the antenna is applied to a handset of broad bandwidth wireless communication system having a large capacitance using a plurality of carrier frequencies, the bandwidth to be provided for the antenna is expanded, and the antenna volume is increased.
Generally, a bandwidth for use in the call between a specific base station and a wireless handset is much narrower than that of the whole system. For each call, the resonant frequency of the antenna is suitably changed to the frequency for use in the call. The bandwidth to be provided for the antenna is reduced, so as to reduce the antenna volume. For this reason, in the slot antenna utilizing a coaxial resonator, Japanese Non-examined Patent Publication No. 11-46115 proposes a tunable slot antenna comprising at least one island conductor provided in a slot, wherein a variable capacitance circuit changes a capacitance value between the island conductor and the wall surface of a conductive cubic, so as to extensively vary the resonant frequency of the antenna. The construction example thereof is shown in FIG. 14.
The antenna comprises a narrow strip conductor 40 disposed in the resonant axial direction of the inner space of a conductive cubic 1 of generally rectangular parallelepiped and so as to be insulated from the conductive cubic 1, and an electric wave transmit/receive slot 2 formed in the top surface of the conductive cubic 1 so as to intersect the strip conductor 40, in which a radio frequency power supply circuit 7 supplies radio frequency power through a power supply conductor 5 and an island conductor 6 between a connection portion 41 set in the strip conductor 40 and the wall surface of the conductive cubic 1.
An island conductor 42 is provided in the slot 2 so as to be insulated from the conductive cubic 1, and a variable capacitance circuit 10 connected between the island conductor 42 and the wall surface of the conductive cubic 1 is connected through a control line 50 to a control circuit 30. The control signal from the control circuit 30 varies the capacity value of the variable capacitance circuit 10, so as to change the capacitance value between the strip conductor 40 and the wall surface of the conductive cubic 1 through the island conductor 42, or between the island conductor 42 and the wall surface of the conductive cubic 1. When the capacitance value between the strip conductor 40 and the wall surface of the conductive cubic 1 is varied through the island conductor 42, the electric current phase on the strip conductor 40 just below the slot 2 is changed. The length of the strip conductor 40 associated with the resonant frequency of the coaxial resonant slot antenna is varied equivalently.
When varying the capacitance value between the island conductor 42 and the wall surface of the conductive cubic 1 as ground potential, for example, if the capacitance value is enough large, the potential of the island conductor 42 is almost equal to the potential of the wall surface of the conductive cubic 1 as ground potential, so as to equivalently reduce the width of the slot 2 by the size of the island conductor 42. Partial reduction of the width of the slot 2 corresponds to an increase in the length of the slot 2. Consequently, the capacitance value of the variable capacitance circuit 10 is varied, so as to equivalently change the length of the slot 2 associated with the resonant frequency of the coaxial resonant slot antenna.
The matching conditions of the tunable slot antenna can be determined by both the electric current phase on the strip conductor 40 just below the slot 2 and the length of the slot 2. Increasing the capacitance value can equivalently increase both the strip conductor length and the slot length, thereby maintaining the matching conditions of the antenna. According to the foregoing principle, the antenna simultaneously and equivalently varies the length of the strip conductor just below the slot and the slot length, so as to extensively change the resonant frequency of the antenna while maintaining the impedance matching state of the antenna.
In the conventional tunable slot antenna proposed in Japanese Non-examined Patent Publication No. 11-46115 described above, since the strip conductor 40 is provided in the conductive cubic 1, the manufacturing process is complex, and the manufacture cost is high.
Further, since the island conductor 42 is provided so as to intersect the strip conductor 42 in the conductive cubic 1, the island conductor 42 must be disposed near the central portion of the slot 2. The variable capacitance circuit 10 connected to the island conductor 42 disposed near the central portion of the slot 2 must be also disposed near the central portion of the slot 2, that is, in the central portion of the antenna. In order to add a control signal to the variable capacitance circuit 10 present in the central portion of the antenna, the control line 50 is provided toward the central portion of the antenna. However, since most electric power is radiated from the slot 2 of the slot antenna, the radiated power is reduced depending on the routing of the control line 50. In Japanese Non-examined Patent Publication No. 11-46115 described above, there is also described a method of applying a DC voltage from one end near the slot 2 of the strip conductor 40 through the through hole and high resistance element to the island conductor 42. According to this method, a control signal can be applied to the island conductor 42 without reducing radiated power. There are, however, the following two problems.
One of the problems is that, when the variable capacitance circuit 10 is a circuit continuously varying the capacitance value with a DC voltage value, and a radio frequency power value inputted to the antenna is increased, the radio frequency power coupled to the island conductor 42 is superimposed on a DC voltage applied to the variable capacitance circuit 10 and the capacitance value is inconstant, and the resonant frequency of the tunable antenna is inconstant. For this reason, when the circuit continuously varying the capacitance value by the DC voltage value is used as the variable capacitance circuit 10, the conventional tunable slot antenna can handle only a small signal, such as a receiving signal of a digital cellular handset.
The other problem is that, when the variable capacitance circuit 10 is a circuit switching between two states of connection/non-connection of the capacitance element by the DC voltage value, the strip conductor 40 also serves as the control line, so that the number of control lines is limited to one, and the resonant frequency realized is also limited to have two values It is thought that one control line can switch a plurality of capacitance elements, but in order to realize this, a complex logic circuit is required, which is disadvantage in view of the packaging density and cost in the case of constructing the wireless handset.
Accordingly, it is an object of the present invention to provide a wireless handset which can package a slot antenna capable of reducing the manufacture cost and packaging cost, and handle a signal of relatively high electric power.
It is another object of the present invention to provide a wireless handset incorporating an antenna, which is small and of high performance for use in a communication system switching a plurality of call frequencies and employing different transmit/receive frequencies.
In order to achieve the foregoing objects, the wireless handset of the present invention comprises a circuit board having a ground plane and a side surface slot antenna packaged in the circuit board. The side surface slot antenna comprises a flat conductive cubic covered with a conductor, a slot having its main portion formed in the side surface of the conductive cubic, a power supply conductor disposed in the slot in the direction intersecting the longitudinal direction of the slot, and a power supply portion for supplying AC power to one of the ends of the power supply conductor.
In a preferred embodiment of the side surface slot antenna, a first island conductor is provided in a portion in which one portion of the slot is extended to the lower surface of the conductive cubic so as not to be in contact with the lower surface of the conductive cubic, and one end of the power supply conductor is connected to the first island conductor, so as to supply AC power through the first island conductor between one end of the power supply conductor and the conductor of the lower surface of the conductive cubic. A variable impedance circuit is connected between the conductor of the single edge of the slot of the top surface of the conductive cubic and the conductor of the lower surface of the conductive cubic in a first position at a constant distant from one of the ends of the slot or opposite ends to the other end along the slot. The variable impedance circuit is connected to a control circuit for varying impedance of the variable impedance circuit.
In a further preferred embodiment of the present invention, when the side surface slot antenna is mounted on the circuit board of the wireless handset, a ground conductor and a plurality of island conductors formed so as not to be in contact with the ground conductor are provided in the surface of the circuit board mounting the slot antenna, the conductor of the lower surface of the conductive cubic is connected to the ground conductor on the circuit board, the plurality of island conductors provided in the lower surface of the conductive cubic are connected to the respective island conductors on the circuit board, so as to supply AC power between the island conductor on the circuit board connected to the first island conductor and the ground conductor on the circuit board, and to connect the first variable impedance circuit between the island conductor on the circuit board connected to the second island conductor, which is formed in the surface of the conductive cubic so as to not to be in contact with the conductor and electrically connected with the single edge of the slot of the top surface of the conductive cubic in the first position, and the ground conductor on the circuit board.
According to the wireless handset of the present invention, an electric wave is radiated efficiently in the direction vertical to the circuit board surface in relation to the ground plane of the circuit board, and the power supply conductor is provided in the side surface of the conductive cubic in the direction orthogonal to the top surface of the conductive cubic. The electric current of the power supply conductor is in parallel with the direction vertical to the top surface of the conductive cubic which is the electric power radiation direction of the side surface slot antenna, and cannot affect the radiated power. Further, since the side surface slot antenna requires no multi-layer construction manufacture process for providing the power supply conductor in the conductive cubic, the manufacture cost can be reduced. In the form for providing the variable impedance circuit at the end of the slot, the impedance is varied so as to vary the resonant frequency of the antenna. In addition, since the position in which the variable impedance circuit is connected is close to the end of the slot, it is possible to dispose a circuit for varying the resonant frequency and the control line for this circuit in the position away from near the central portion of the slot having the largest radiated power of the antenna, without greatly affecting the radiated power.
In order to achieve the objects of the present invention, the wireless handset of the present invention for use in a communication system switching a plurality of call frequencies and employing different transmit/receive frequencies, comprises a circuit board having in its interior a ground plane, and slot antennas of the present invention for transmit and receive, independently, wherein the transmit/receive slot antennas are integrally formed by interposing a support therebetween so as to align the directions of the main polarizations, and then the integration is mounted such that the lower surface of the conductive cubic of each of the transmit/receive antennas is directed to the surface of the circuit board facing the opposite side of the user when using the wireless handset.
According to the wireless handset of the present invention, since the volume is typically proportional to the bandwidth in the antenna, as compared with the volume of the antenna having a bandwidth covering all the transmit/receive bandwidths provided by interposing the transmit/receive isolation bandwidth, the volume of the antenna covering the transmit/receive bandwidths can be reduced by more than half. The total antenna volume can be reduced, so that the wireless handset incorporating the antenna can be smaller. The directions of the main polarizations of transmit/receive antennas are aligned in the direction of the polarization for use in the system employing the wireless handset mounting the antenna, whereby transmit/receive can efficiently be performed, Since the distance between the transmit/receive antennas can be maintained constant by the support, an amount of isolation between the transmit/receive antennas isolated is constant regardless of how to mount the antenna, thereby giving stable properties.
Since the slot antenna of the present invention is a magnetic current type antenna, the slot antenna is mounted on the surface of the circuit board having in its interior a ground plane facing the opposite side of the user when using the wireless handset, whereby electric power can effectively be radiated to the side opposite the user. In addition, it is possible to package the circuit in the wireless handset case of the wireless handset user side viewed from the antenna, such that the packaging density can be high so as to make the wireless handset smaller. Since the side surface slot antenna is of single-layer flat construction, the transmit/receive slot antennas and the support can easily be integrally formed, so that the manufacture cost can be reduced as compared with the case where the transmit/receive antennas are manufactured independently to be combined. When the transmit/receive antennas are connected by the support such that the magnetic currents are aligned in a straight line, the coupling between the transmit/receive antennas is minimum, and it is possible to reduce leak of a signal from the transmit radio frequency circuit to the receive radio frequency circuit in the wireless handset for performing transmit/receive at the same time.
Other features and effects of the wireless handset of the present invention and the preferred embodiments of the side surface slot antenna and the effects thereof will be described in detail in the following embodiments of the present invention.